The stock RB blocks, all of the passenger car production blocks, have thinner main webbing than any of the stock 400 blocks came with
I've seen 5 RB blocks with cracks in the main webbing between #1 and #3 cylinders, no cracks in any 400 block so far
If your going to make a Mopar BB race motor that makes north of 750 HP on gasoline buy a new race block to start with